The present invention relates to a process for the preparation of aromatic isocyanates wherein aromatic carbamates (referred to hereinafter as the carbamates) are pyrolyzed by bringing them into contact with a catalyst dissolved in a solvent inert to isocyanates.
Isocyanates are very useful substances chiefly as starting materials for polyurethanes. In particular, tolylene diisocyanate, methylene-bis-(4-phenyl isocyanate) and naphthylene diisocyanates are now prepared on a large commercial scale.
A current process for preparing these isocyanates, for example, tolylene diisocyanate of the formula: ##STR1## comprises nitrating toluene to form dinitrotoluene, reducing the latter with hydrogen to form the corresponding diamine and then reacting the diamine with phosgene. Namely, the current process comprises complicated and troublesome steps, requring the use of a large amount of highly toxic phosgene and permitting the formation of hydrogen chloride as by-product. In this field, therefore, there is a great demand and research for a new and improved process which requires no phosgene and is simpler and more economical than the above process.
One process which might be considered as a substitute for the current process employing phosgene is a process for preparing isocyanates wherein carbamates are pyrolyzed. Such a process, which relatively easily synthecizes carbamates directly from mitro compounds was developed in recent years. However, the known conventional pyrolysis process is industrially inoperable or economically disadvantageous in respect of yield of isocyanates, reaction rate, materials of construction, control of temperatures and elimination of by-products, and is consequently not practiced as a suitable process for preparing isocyanates.
It has now been found that the above mentioned drawbacks can be overcome when carbamates are subjected to a liquid phase pyrolysis under reduced pressure in the presence of specific catalysts.
The reaction for forming isocyanates by pyrolysis of carbamates may be shown by the following basic equation: EQU RNHCO.sub.2 R' .fwdarw. RNCO + R'Oh (1)
On thermal dissociation of the carbamate, several undesirable side reactions take place at the same time. These side reactions are: the decarboxylation reaction of the carbamate accompanying the formation of a primary amine RNH.sub.2 and an olefin or of a secondary amine RNHR' as by-product; the reaction between the produced isocyanate and the starting carbamate, permitting the formation of an allophanate as by-product; the reaction between the produced isocyanate and an amine formed as by-product permitting the formation of a urea compound as by-product; and the polymerization of the produced isocyanate, permitting the formation of an isocyanurate or a polymer as by product. The thermal dissociation reaction of equation (1) above is reversible and its equilibrium remains with the left-hand side carbamate at normal temperature but is shifted to the right-hand side by heating, whereby the dissociation of the carbamate takes place. In this case, the thermal dissociation temperature varies according to the sort of carbamate and the reaction conditions. Accordingly, it is important for obtaining isocyanates advantageously from carbamates to perform the pyrolysis reaction of equation (1) selectively while inhibiting the above mentioned side and reverse reactions.
The conventional pyrolysis of carbamates is roughly classified into reactions carried out in the vapor phase at a high temperature and reactions carried out in the liquid phase at a relatively low temperature. U.S. Pat. No. 3,734,941 discloses a typical vapor phase process wherein a carbamate is pyrolyzed at 400.degree.-600.degree. C in the presence of a Lewis acid and the resultant vapor is separated by fractional condensation into an isocyanate and an alcohol. According to this process, for example, tolylene diisocyanate is obtained in a yield of 60% by pyrolysis of diethyl tolylene-2,4-dicarbamate of the formula: ##STR2## in the presence of ferric chloride. However, this process has the drawbacks of a low yield of the product, decomposition of the catalyst, corrosion of the reaction apparatus at high temperatures, and formation of a considerable amount of a polymer as by-product. German Pat. No. 2,410,505 proposed as an improved vapor phase method a process wherein the residence time of the reactants at 350.degree.-550.degree. C is controlled within 15 seconds. According to this improved process, the yield of isocyanate as high as 93%, although the carbamate has to be supplied in the form of powders to the reaction zone. However, a solid polymer is also formed by this improved process as by-product and is gradually deposited in the reactor and in the condenser during the course of sustained operation, thus making it difficult to conduct a continuous reaction. In addition, a large quantity of heat required for the endothermic pyrolytic reaction has to be supplied to the starting material within a very short period of time. This additional factor causes this improved process to encounter great difficulty in being adopted into practice.
If a liquid phase pyrolysis of the carbamates could be performed at a high reaction rate to afford the end product in a high yield and at a temperature lower than that adopted in the vapor phase methods, elimination of by-products and supply and control of the heat of reaction would become easier and the process as a whole would become very advantageous. According to U.S. Pat. No. 2,409,712, a reaction mixture containing ethoxyethoxyethyl N-laurylcarbamate, synthesized in a yield of 57% by heating laurylamine, urea and ethoxyethoxyethanol at 200.degree. C for 3 hours, is subjected directly to liquid phase pyrolysis conducted at 210.degree.-230.degree. C under reduced pressure of 2 mmHg whereby lauryl isocyanate is isolated in a yield of 75%. This fact shows that a liquid phase pyrolysis of carbamates to isocyanates takes place relatively easily. However, such yield is still too low to be practical. This is due to the reason that in the case of liquid phase pyrolysis of carbamates in the absence of a solvent or in the presence of a solvent containing even such a substance (reactive with isocyanates) as mentioned above, the concentration of the reactants, i.e. the concentration of functional groups such as OH, NCO and NHCO.sub.2 R becomes extremely high and the reaction time also increases when compared with the above mentioned vapor phase pyrolysis, so that the various above mentioned side reactions tend to take place and this tendency is more noticeable than the inhibition of the side reactions by lowering the temperature. It is known already to promote thermal dissociation while inhibiting side reactions by diluting the reactants with an inert solvent to lower the concentration of the functional groups.
It has been reported that, as a result of performing the pyrolysis of carbamates in the presence of various amines or fatty acids in an inert solvent such as a hydrocarbon, ether or nitrobenzene, the rate of thermal dissociation is increased as the acidity of alkalinity becomes stronger or as the polarity of the inert solvent becomes higher. [Mukaiyama et al., J. Amer. Chem. Soc. 78, 1946), Bull. Chem. Soc., Japan 33, 1137 (1960)]. It has also been reported that, as result of measuring the thermal dissociation temperatures of various carbamates during the thermal dissociation reaction of various carbamates to tolylene diisocyanates in an inert solvent selected from a paraffin oil and methoxypolyethylene glycol, the thermal dissociation temperature is lower in the case of using the latter inert solvent [G. R. Griffin et al., I & EC Product Research and Development 1, 265 (1962)]. It is thus evident from these reports that a thermally stable solvent with high polarity is desirable as an inert solvent for pryolysis of carbamates.
German patent publication DOS No. 2,421,503 discloses a method of separately recovering isocyanates and alcohols by dissolving carbamates in an inert solvent such as hydrocarbon, ether, ketone or ester and pyrolyzing the carbamates at 175.degree.-350.degree. C under atmospheric or superatmospheric pressure in the presence of a carrier. Carbamates shown in 20 examples of this publication are those capable of forming phenyl isocyanate, tolylene diisocyanate and hexamethylene diisocyanate. The yield of the isocyanate products obtained by separating alcohols formed by thermal dissociation of the carbamates is 22-84% in all examples. The synthesis rate of the isocyanate products is 23-84 g/liter per hour in 6 examples wherein the yields of the isocyanate products are at least 70%. The best yield is obtained in an example wherein tolylene diisocyanate is obtained by pyrolysis of tolylene-2,4-dicarbamate. In this example, the reaction is carried out by continuously supplying n-hexadecane and a solution of the dicarbamate in tetrahydrofuran into a flask charged with n-hexadecane as inert solvent, pyrolyzing the dicarbamate at 250.degree. C under slightly superatmospheric pressure while blowing a large amount of nitrogen as carrier into the flask and recovering the pyrolyzed fraction distilled at 180.degree. C over the top of a fractionating column fitted to the flask after an average resident time of about 20 hours. In a steady stage, the end product, i.e. tolylene diisocyanate is recovered at a yield of 84% and a monocarbamate at a yield of 9%. In this case the synthesis rate of the diisocyanate is about 21 g/liter per hour. The yield obtained in this method is higher than that of the aforementioned U.S. Pat. No. 2,409,712 but is still too low to be practical. In addition, a problem arises in that the reactants are diluted with a solvent so as to decrease the synthesis rate of the isocyanate (the yield of the isocyanate aimed at per unit volume per hour). Accordingly, the isocyanates and the carbamates which easily undergo the above mentioned various side reactions have to be maintained at a high temperature for a long period of time, thus causing a decrease in the yield of the end product. Further, a considerably large capacity is required, thus making the process economically unacceptable. In this liquid phase process, therefore, the yield of the product is not satisfactory and the reaction rate is too small.
The above reference discloses that an acid such as a fatty acid or sulfuric acid or a base such as an amine functions as a catalyst for the thermal dissociation reaction in the liquid phase of carbamates to isocyanates and that the reaction is promoted more smoothly as the acid or base becomes stronger. As such acid or base reacts with the resultant isocyanates, however, the acid or base can no longer be used as catalyst.
Recently a process for preparing isocyanates has been described in U.S. Pat. No. 3,919,278 wherein a mononuclear aromatic carbamate is dissolved in an inert solvent in an amount such that the total concentration of the carbamate and a product obtained by pyrolysis thereof is within a range of about 1-20 mol % and the pyrolysis of the carbamate is carried out at 230-290.degree. C in the presence of an inert carrier used in an amount of at least 3 molar proportion to the carbamate. Another process for preparing isocyanates is described in U.S. Pat. No. 3,919,279 wherein a carbamate is dissolved in an inert solvent and brought into contact at a high temperature with a catalyst composed of a heavy metal (Mo, V, Mn. Fe, Co, Cr, Cu or Ni) or a compound thereof to effect the pyrolysis of the carbamate. The former process (U.S. Pat. No. 3,919,278) is almost identical to that disclosed in the above mentioned German DOS No. 2,421,503, and requires the use of a large amount of a carrier and can hardly be carried out under subatmospheric pressure. The latter process (U.S. Pat. No. 3,919,279) although it uses as a catalyst a specific metal or a compound thereof, is limited to using such catalyst under the reaction conditions in the presence of a carrier under atmospheric or superatmospheric pressure and so cannot be carried out under subatmospheric pressure.